Ya I am not sure. I don't need a vorlauf, but have never seen a loss of gravity points on the OG (I actually have come out a bit higher in most cases). My grain is loose in the bag... the bag is spread out lining the bottom and sides of the mash tun, not compressing the grain (its not suspended, just sits on the bottom of the tun). I took the idea from the Brulosophy guy who I believe also tested against not using the bag (testing OG, FG, and triangle taste tests etc.) I have not found a reason not to use the bag as a strainer so far, so until I do or brulosoophy determines a detectable negative difference, it will be my SOP. I tend to fly sparge pretty slowly as well.